I have an HVAC geo-thermal unit that I post on elsewhere on the geo forum, but I think that Air flow is a single, important subject that is not well understand by homeowners and is rarely mastered by 70% of installers and HVAC technicians, certainly in my state. I am a commercial HVAC technician who also owns his own geo-thermal heat pump. The last two weeks I paid for a two part class (or review) of air flow principles and current news about ECM motors. I am going to share both what I know and recently reviewed. Apart from you engineers, we'll skip the parts about latent heat removal, MERV 8, or things that go over most people's heads. I want to talk about a couple of things that are essential FOR EVERYONE to do, and know, and remember in plain language. 1) Manufacturers do not "preset air flows on brand new equipment from the factory. Strictly speaking, your brand new unit IS NOT ready to "plug in and go". Get that straight ! 2) 96% of so called installers and HVAC technicians really do not understand air flow and NEVER measure it during an install or reluctantly measure it afterwards to understand and fix problems. You might want to think about that. 3) A house can be 100 years old, 30 years old, or brand new construction. When people decide to reuse pre existing ductwork, how do they know the original was sized correctly (LARGE ENOUGH) or isn't leaking like a sieve ? Often HVAC installers like to "sell on the cheap" because supposedly "price is everything". Great ! Suppose the existing return ducts and supply ducts have 1/2" of dust, mildew, and dust mites in it ? Are you just going to just "ram" a new system in there ? Mr. Customer, you get what you pay for ! When I changed out my system in 2008, I had to take into account that my eight year old special needs daughter frequently caught pneumonia. Melodie got pneumonia every year. She was fragile. When we pulled out a 25 year old furnace, the ductwork was exposed. The dust alone was 1/2" thick, particularly in the returns. I had to say to the installing company, WHOA ! Let me figure out what to do to clean (otherwise sound metal ductwork) Since the unit was physically removed, it was easy to call Stanley Steamer. They came in. They installed a vacuum backer plate on the ductwork. They brought their twirly brushes in and brushed out the ductwork as their powerful vacuum sucked out all the crap, and my special needs daughter breathed easier, as the new unit was installed. I am not doing commercials for SS, but please figure out what you are going to do ! A sharp company would measure air flow and static pressure on the original unit to have an idea or clue as to whether there are pre existing problems like RESTRICTIONS. A typical scenario goes like this: installer bids job cheap, ductwork is fouled up, no $$ or will to fix it or install new ductwork; result ? One recipe for an unhappy customer. 4) Every installing company is supposed to measure total static pressure across the system, and actual running airflow. All modern air handlers have 4 or 5 speeds that can be set for airflow. So now do you set the system on tap one or tap five ? Few do this. But why do we have to measure airflow ? Answer: Because it tells you if you have restrictions on the supply side or the inlet side. And if air flow or ductwork is not right, the customer (that's you !) will be unsatisfied and unhappy. Most systems are rated for .5 " static pressure. That is what we are shooting for. All too often we never reach it, and sadly too many ""installers/technicians"" stick out their hand in front of a supply vent and say: "yeah air flow is good", which would be like me taking a wild guess and arbitrarily prescribing the dosage for your blood pressure medicine, in other words totally with out informed knowledge. It is likely that these days, newer ductwork may be more correctly sized, avoiding some of these problems, but the fact is many houses are sized without the official "MANUEL J" calculations, and people who should know better, guess at ductwork sizes instead of using a ductilator slide calculator or an app that can be installed on a I-pad or I-phone. The biggest problem is UNDERSIZING. 5) Good News ! There are many new, great, accurate instruments sold at HVAC houses that can measure wet bulb temperatures, dry bulb temperatures, pressure drop, and air flow. If you are an installer or technicians and don't feel they need to be in your toolbox, GET OUT OF THE TRADE. You're giving the rest of us a "bad name" 6) You're boring me. That's too much to do ! Well then consider this: "If air flows are too slow, the condenser has to get rid of more heat. You also will have a higher compression ratio. (Great you say, the customer will feel warm in winter) WRONG, it will cause shorter coil life and contribute to earlier coil FAILURE ! Have I finally gotten your attention ? The best way to do a start up is to start on medium high or high and then adjust down, BUT DO THE DAM MEASUREMENTS ! The standard rule is you need 400-450 cfm per ton. If your charts say the units need 950 cfm, and you measure 700 cfm, you have a problem, make the ECM motor go faster. Tip: You are more likely to cause a problem or hurt the unit with too low air flow than the opposite, but the point is set the unit to the air flows that the manufacturer requires. An installer incapable of doing this is incompetent.